1. Field of the Present Invention
The present invention relates to an exhaust gas purifying apparatus for an internal combustion engine in which a driving force is assisted by an electric motor, and particularly to an exhaust gas purifying apparatus for an internal combustion engine having an exhaust system which is provided with a nitrogen oxide (NOx) purifying apparatus containing an absorbent for NOx
2. Description of the Related Art
When the lean operation in which the air-fuel ratio of an air-fuel mixture which is to be supplied to an internal combustion engine is set to be on the lean side with respect to a theoretical air-fuel ratio is executed, the discharged amount of NOx tends to be increased. Conventionally, it has been known that a NOx absorbent that absorbs NOx is incorporated into an exhaust system of an engine so as to purify an exhaust gas. Such a NOx absorbent has properties that, in a state where the air-fuel ratio is set to be on the lean side with respect to a theoretical air-fuel ratio and the oxygen concentration in the exhaust gas is relatively high (NOx is rich) (hereinafter, referred to as "exhaust gas-lean state"), the absorbent absorbs Nox. Alternatively, in a state where the air-fuel ratio is set to be in the vicinity of the theoretical air-fuel ratio or on the rich side with respect to the theoretical air-fuel ratio, the oxygen concentration in the exhaust gas is relatively low (hereinafter, referred to as "exhaust gas-rich state"), the absorbent releases NOx which has been absorbed. A NOx purifying apparatus containing such a NOx absorbent is configured so that, in the exhaust gas-rich state, NOx released from the NOx absorbent is reduced by HC and CO and then discharged as a nitrogen gas, and HC and CO are oxidized and then discharged as water vapor and carbon dioxide.
The NOx absorbent has naturally a limit on the absorbable amount of Nox. Hence, it is impossible to continue only the lean operation for a long term. In the conventional art, therefore, an air-fuel ratio controlling technique is known in which the air-fuel ratio is temporarily enriched in order to release NOx from the NOx absorbent, NOx is then released from the NOx absorbent, and the released NOx is reduced (for example, Japanese Patent No. 2,692,530). Hereinafter, this temporary enrichment is referred to as "reduction enrichment".
In the technique disclosed in the Japanese patent, when an opened state of a throttle valve is to be transferred to a fully closed state, the air-fuel ratio is changed from a value on the lean side with respect to the theoretical air-fuel ratio to the theoretical air-fuel ratio. However, immediately after an engine is transferred to an idling state, reduction enrichment in which the ratio is set to a value on the rich side with respect to the theoretical air-fuel ratio is performed for a short time period. This is performed in order to, for a short time period, reduce NOx which has been absorbed in the NOx absorbent during the lean operation. When the fuel supply is stopped during deceleration of the engine, a control is performed for the same purpose in the following manner. Reduction enrichment is executed at timing when the fuel supply is restarted immediately after the stop of the fuel supply. Thereafter, the air-fuel ratio is transferred to the theoretical air-fuel ratio.
In a so-called hybrid vehicle having an internal combustion engine and an electric motor which assists the driving force of the internal combustion engine, the vehicle can be driven by a driving force exerted by the motor. Accordingly, when a throttle valve of the internal combustion engine is fully closed, a control is sometimes performed so as to stop the engine. In some cases, therefore, reduction enrichment cannot be executed immediately after the engine is transferred to an idling state as the above-described conventional art. In such cases, the NOx absorbing ability of the NOx absorbent may be lowered, so that the exhaust gas properties are impaired.